1. Field of the Invention
The present invention concerns a method for determination of a diffusion-weighted image of an examination subject in a magnetic resonance system and a magnetic resonance system for this purpose. The invention in particular concerns a method and system of the type wherein the diffusion-weighted images are acquired using a double spin echo imaging sequence.
2. Description of the Prior Art
In magnetic resonance tomography imaging methods have been established with which the diffusion of the water molecules in the human body can be made visible. With the aid of diffusion imaging, magnetic resonance tomography in particular supplies valuable insights in the assessment of patho-physiological events and processes, for example in the human brain. It is in particular possible to localize stroke areas in the brain.
The difference arises due to the thermal translation movement of molecules. This is a random process that is also known as Brownian molecular motion. The distances covered by the diffused molecules on which the diffusion-weighted magnetic resonance tomography is based are very small. Strong magnetic field gradients (known as diffusion gradients) are used for diffusion-weighted imaging. These strong magnetic field gradients are used in addition to the gradient fields in the imaging sequence that are necessary for spatial coding. For diffusion imaging the physical effect is utilized that the magnetic resonance signal decreases given a difference movement of the molecules under the influence of magnetic field gradients. The phase of the magnetic resonance signal changes in proportion to the applied magnetic field gradients. If a bipolar diffusion magnetic field gradient is now activated, the phase development for resting (non-moving) spins now cancels out due to the bipolar gradient; for spins moving due to diffusion the moving spin experiences a different magnetic field. In the case of a bipolar diffusion gradient, for diffusing spins the signals at the end of both diffusion gradients are no longer phase coherent (as for resting spins) but rather are distributed. This entails a signal decrease. If a non-diffusion-weighted data set and a diffusion-weighted data set (given which additional diffusion gradients are activated) are now acquired, a conclusion about the diffusion can be reached from the difference of the two data sets.
The diffusion is typically a diffusion tensor that depends on the spatial direction. This diffusion tensor is a symmetrical tensor with six coefficients. To determine the diffusion tensor, non-diffusion-weighted data sets and at least six (for example 20-60) diffusion-weighted data sets are acquired in order to calculate the diffusion tensor.
Diffusion-weighted imaging techniques are very sensitive to gradient-induced eddy current effects. To avoid eddy current effects it was, for example, proposed in the conference volume of the Society of Magnetic Resonance in Medicine, page 799 in the year 2000, to use a double spin echo sequence with four diffusion gradients, i.e. two diffusion gradient pairs.
Furthermore, in such double spin echo sequences gradients known as spoilers or dephasing gradient fields are applied in order to avoid unwanted ancillary echoes. In spite of this, stripe artifacts in the magnetic resonance image have occurred under specific circumstances with such diffusion-weighted imaging sequences.